Viscoelastic properties of thermo-hydro-mechanically treated beech (Fagus sylvatica L.) determined using dynamic mechanical analysis

Abstract

Thermo-hydro-mechanical (THM) processing can improve the intrinsic properties of wood, produce new materials, and give desired form and function to new applications. THM treatments change the mechanical properties of wood and may change its viscoelastic properties as well. Therefore, the objective of this study was to assess the viscoelastic properties of THM-treated wood at several humidity and load levels. To explore these changes, this study applied a THM treatment to beech (Fagus sylvatica L.) wood with steam (620 kPa) and heat (170 °C), followed by densification and increased temperature (200 °C) in a hot-press, which was then cooled while under pressure. Two initial specimen thicknesses before THM treatment were used to study the difference between density ratios. Specimens were tested in a humidity-controlled dynamic mechanical analyser (DMA) to apply creep stress with different loading levels (20% and 30% of expected modulus of rupture) and relative humidity levels (30%, 50%, and 65% RH). The creep compliance/recovery response was monitored, and dynamic moduli were measured before and after the application of creep stress. The loss modulus measured was highest for specimens tested at 65% RH and lowest in specimens tested at 30% RH, which is a direct result of the viscous response of the material. Increased damping of the specimens was also observed at higher RH, which is typical for wood products due to added moisture in the cell wall acting as a plasticizer to cell wall polymers. Like previous studies, THM treatment lowered the equilibrium moisture content (EMC) of densified wood specimens, which affected their mechanical performance. THM treatment yields higher strength and lower EMC, suggesting that this product could be suitable for structural applications where their service life is in an indoor environment. THM treatment also resulted in decreased creep compliance and recovery compliance as compared to control specimens.

Creep Properties of Densified Wood in Bending

“…Compared to the untreated R group, the creep compliance of all the modified groups (D, RI-D, and D-TM) was reduced by 40-55% after 14 days, which means that the densification reduced the creep deformation and that both resin impregnation and thermal modification enhanced this effect. This result agrees with a previous study of the viscoelastic properties of densified wood using dynamic mechanical analysis, which showed a lower creep compliance of densified wood under controlled RH [39]. None of the specimens reached the tertiary creep stage, where the deformation rapidly increases and leads to failure of the material.…”

Investigations of the Chemical Distribution in Sorbitol and Citric Acid (SorCA) Treated Wood—Development of a Quality Control Method on the Basis of Electromagnetic Radiation

“…It also appears that the solution uptake was higher for earlywood (dips) than for latewood (peaks), as indicated by a higher shift of the density profile's curve in the y-axis direction. Since the earlywood is characterized by a higher porosity than latewood [59], it facilitates higher volume uptake, which could explain the higher deposition of the impregnation chemicals. A similar observation has been made for a linseed oil-treated wood [39].…”

Selected Properties of Densified Hornbeam and Paulownia Wood Plasticised in Ammonia Solution

“…This is due to the structural diversity and composition of the chemical cell walls of individual types of wood. Species with a lower density can be densified more easily than species with a higher value [ 15 , 16 ]. This is related to porosity and susceptibility to deformation.…”